I've owned probably a dozen or so over the years. I's, II's, III's, 22/45's...pick one. I couldn't find a dimes worth of practical difference. One worked just as well as the other. I've never even seen a IV, (I don't look anywhere but the used gun case. I'm sure my dealer has them.) I didn't get all excited about the IV because I consider it a solution looking for a problem. I never had any trouble taking down the old ones, when I wanted to, which wasn't often. I'd spray them Gunscrubber, run a patch through the bore, put a little oil on/in them and let them go. I can't say one never malfunctioned, but it wasn't often enough to take note of, didn't matter what type of ammo. My favorite one was just the standard pistol with a 4" tapered barrel and fixed sights, although I had a 22/45 with a 4" bull barrel, and a cheap BSA red dot that would just shoot lights out. Except for the sight, stock out of the box.
I don't have one now, but that's only because I sold off most of my accumulation and am down to just one 22 these days, and it's a Model 18 S&W.